The proposed study is aimed toward further development of a novel gene delivery systemusing lentiviruses as vehicles. The establishment of a lentiviral vector system will complement the deficiency of the conventional murine leukemia retrovirus-based vectors and broaden the target cell population to include nondividing cells. This vector system can be used for the delivery of therapeutic genes into hematopoietic stem cells and differentiated tissues and organs for the treatment of acquired and inherited diseases in humans. The proof of principals has been presented recently. However, a convenient and safe lentiviral vector system has not been established. Inparticular, issues regarding the safety ofusing lentiviral vectors have not been carefully investigated. In this grant proposal, the following specific aims are proposed in order to assist the establishment of and to evaluate the safety of the lentiviral vector system: 1) To develop a lentiviral vector system using attenuated recombinant human immunodeficiency virus (HIV) type 1, HIV type 2 an dsimian immunodeficiency virus (SIV) constructs; 2) To examine the effects of different lentiviral sequences and gene functions on vector packaging and transduction efficiencies; 3) To study the transduction efficiencies and the term of expression of vesicular stomatitis virus G glycoprotein envelope pseudotyped HIV-1, HIV-2 and SIV vectors in proliferating and nonproliferating cells in tissue culture and in small animal models; 4) To evaluate the cytotoxicity and the host immune responses associated with lintiviral transductions in tissue culture an dhumanized severe combined immunodeficiency (SCID/beige) mice; 5) To examine the possibility of generating replication-competent HIVs during vector preparation and transduction of human cells, and to test the replication potential of an artifical HIV/human endogenous retrovirus recombinant in tissue culture and in humanized SCID/beige mice. Further understanding of lentiviral vectorology and studies into its in vivo immunity will provide important information towards the application of lentiviral gene delivery tools in human gene therapy trials.